A-320 MISSES XING ALT RESTR ON DSCNT.

Narrative

IFR MKE TO DTW. CTR CLRED US DIRECT POLAR; POLAR 1 ARR INTO DTW; TO CROSS 60 MI W OF POLAR AT FL230. CAPT WAS PF AND WAS TRYING TO ENTER XING RESTR INTO MCDU. AFTER 2 UNSUCCESSFUL ATTEMPTS HE STARTED A DSCNT LEAVING FL290 TO FL230 USING VERT SPD. DURING THIS TIME I WAS USING THE PA TO GIVE OUR PAX AN ARR BRIEFING. DURING MY PA I OVERHEARD ZOB ASKING WHAT OUR DISTANCE TO POLAR WAS. THE CAPT STATED 59 DME (AND OUR ALT WAS FL258). THE CAPT WAS USING FULL SPD BRAKES AT THIS TIME TO HELP REACH FL230; BUT WE DIDN'T REACH FL230 UNTIL APPROX 52-55 DME FROM POLAR. WE MISSED THE XING CLRNC BY ABOUT 3000 FT BUT THERE DID NOT SEEM TO BE ANY CONFLICTS AT THAT TIME. CONTRIBUTING FACTORS WERE THE CAPT'S FIXATION AND ATTEMPTS TRYING TO PUT THE XING RESTR INTO THE MCDU INSTEAD OF USING VERT SPD OR OPEN DSCNT. ANOTHER FACTOR IS MY GIVING A PA TO OUR PAX DURING A SOMEWHAT CRITICAL PHASE OF FLT. THERE IS ALSO NO WAY TO PROGRAM A XING RESTR INTO OUR MCDU UNLESS IT FALLS ON A PARTICULAR INTXN; VOR; ETC. OTHER THAN THAT; WE MUST BUILD IT OURSELVES AND IN SOME CASES (SUCH AS THIS ONE) IT WOULD NOT ACCEPT IT.